Oh, goodness!  I didn't mean to imply I felt sure it isn't a lactuca.  It's just that I spent hours and hours looking at photographs of every lactuca I could find on-line or in books -- and none of 'em looked anything like it.  No, I'm not sure at all -- and, yes, the other characteristics are persuasive.  I dunno!  It's a mystery -- and it's a mystery I'd like (I suppose we'd both like) to solve.
 Yes, Angelica, there are many d'lion look-a-likes in Oz + dandelions.  The seeds and some other features of our mystery plant are radically unlike d'lion though.
 In any case, I'll keep looking and will, of course, post anything useful or definitive I find.
 Best regards from the Atherton Tablelands,
 Randy